Can someone please explain what the line for(;Q.size();) does?

Question

I am trying to understand a source code and i cannot figure out how the line for(;Q.size();) is meant to work. Could someone please simplify it for me ?

It's the same as `while(Q.size())`. – Blaze Mar 02 '20 at 07:51 — Blaze, Mar 02 '20 at 07:51
Or more clearly, `while(!Q.empty())` – MSalters Mar 02 '20 at 08:08 — MSalters, Mar 02 '20 at 08:08

score 2 · Answer 1 · answered Mar 02 '20 at 08:02

A for statement consists of three parts, separated by semicolons:

an init-statement
a condition
an iteration_expression

A for loop is equivalent to this code:

{
    init_statement 
    while ( condition ) { 
        statement 
        iteration_expression ; 
    }
}

The init-statement and iteration_expression can be empty, but the semicolons between them are still required.

In your example, for(;Q.size();) would thus be equivalent to:

{
    while ( Q.size() ) { 
        statement 
    }
}

score 1 · Answer 2 · answered Mar 02 '20 at 07:56

1

Look at it this way:

for(<do nothing>;Q.size();<do nothing>) {//do something}

Now read the definition of the for loop and see that it fits perfectly. As mentioned by others, essentially this becomes equivalent to while(Q.size())

answered Mar 02 '20 at 07:56

SomeWittyUsername

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score 0 · Answer 3 · answered Mar 02 '20 at 07:54

It's a for loop which doesn't care about an incrementing index variable. As Blaze pointed out it's equivalent to a while loop.

for(;Q.size();)
{
   // do something while Q is not empty
}

or equivalently

while(Q.size())
{
   // do something while Q is not empty
}

Can someone please explain what the line for(;Q.size();) does?

3 Answers3